IntroductionHad planet Earth been named by extraterrestrials, they would undoubtedly
have named it the Water Planet (planet Aqua or Oceanus) because nearly
71 percent of its surface is covered by salt water. Although very noticeable
from space, the glaciers and ice caps contain only 1.6% of the total amount
of water. Were all ice to melt, (assuming all ice is above sea level),
the ocean (which is 3800 m deep, on average), could rise by 1.6 x 38 =
61 m. In past ice ages, the ocean stood about 90m lower (at some time even
125m!), implying that the amount of water as ice could have been
as much as 4%.

Where is all the planet's water?

97.957% in the oceans 1.641% in glaciers and ice caps 0.365% in ground water 0.036% in lakes and rivers 0.001% in the atmosphere

The view of the world that includes most of the land, still
contains a large share of the oceans. In this part of the globe, live over
90% of all people. It is also where most environmental problems are felt.
In the centre one sees the Atlantic Ocean and east of Africa the Indian
Ocean, looking much smaller due to the curvature of the globe.

The view of the water world, contains very little land and
few people. The main continents of North and South America and Asia, are
just visible around the edges. New Zealand is the continent, that lies
closest to the centre of the water world, the Pacific Ocean. West of Australia
the Indian Ocean.

Because most of the land is found in the Northern Hemisphere, the two
hemispheres behave very differently in response to annual variation in
solar radiation. The stabilising effect of the oceans makes that the Southern
Hemisphere has smaller temperature differences between winter and summer,
resulting in less wind and more temperate winters and summers (See Temperature,
below). It may also explain why the large Pacific Ocean is more tranquil
than the smaller Atlantic Ocean.

Distribution
of surface areaMost maps of the planet show height contours. These are hypsographic
maps (Gk
hypsos
= height). When all height contours of both sea
and land are put together, the hypsographic curve results as shown here.
It gives an immediate overview of the distribution of height and depth
over the planet. Horizontally is the surface area of the planet as a percentage
of the total (5.2 million km2).
The seas occupy just over 70% and the land just under 30%. The amount of
land above sea level (brown) is very much less than the volume of sea (blue).
If the land was spread into the sea, the oceans would still be 3000m deep!
The highest point on land is Mount Everest (8848m). The deepest point in
the ocean is in the Mariana Trench (11034m)

The diagram gives us an idea of the general shape
of the ocean basin. Only a very small part goes deeper than 6000m and the
deepest troughs of 10km are hardly visible here, likewise for the land
and its tallest mountains. The most important bits are almost invisible
on this diagram: the land up to 500m where most of the people live and
the sea down to 200m where most of the ocean's productivity is found. From
this small margin of about 12 + 5 = 17%, the world population feeds itself.
Click here for a larger diagram.

In
the lefthand margin of the hypsographic curve above, a histogram is shown
of the distribution of height of Earth's surface, and this curve is shown
in more detail here. The zero axis (red line) is sea level but not average
height. Since the Magellan space probe mapped the landscape of Venus
accurately in 1990, the two surface areas can be compared. The red curve
is that of Venus, centred around its average height because Venus has no
oceans. It is a bell shaped curve as can be expected from any planet, but
it differs remarkably from Earth's.
Either the oceans have carved a deep dent into the smooth bell of Earth's
crust, or we must assume that the crust consists of two separate parts,
the continents and the ocean basins. As we shall see later, this is the
case.

planet

distance
from sun
million km

diameter
km

mass
relative
to Earth

rotation
period

orbit
period

mean
temp ºC

atmosphere
and ocean

Mercury

57.9

4878

0.055

58.6d

0.24y

-170 to 430

Na

Venus

108.2

12104

0.815

-243d

0.62y

-23 to 480

CO2+N

Earth

149.6

12756

1.000

23.94h

1.00y

16

N+O+H2O

Mars

227.9

6787

0.107

24.62h

1.88y

-50

CO2+N+Ar

Jupiter

778.3

142800

317.8

9.93h

11.86y

-150

H+He+CH4+NH3

Saturn

1429

120000

95.2

10.5h

29.48y

-180

H+He+CH4+NH3

Uranus

2875

50800

14.5

-17.24h

84.01y

-210

H+He+CH4

Neptune

4504

48600

17.2

16h

164.8y

-220

H+CH4+He?

Pluto

5900

2245

0.002

6.4d

247.71y

-230

CH4?

Outstanding differences
have been marked in red

Origin of the oceansThe origin of the oceans has puzzled people for a very long time, and
even today, the issue has not been completely settled. The biblical view,
expressed in the chapter Genesis, has held until last century, when other
theories began to surface as a result of scientific knowledge. We'll have
a look at some of these.

The biblical perspective

GENESIS1:1 In the beginning God created the heaven and the earth1:2 And the earth was without form, and void, and darkness
[was] upon the face of the deep. And
the Spirit of God moved upon the face of the waters.1:3 And God said, Let there be light: and there was light.1:4 And God saw the light that [it was] good and God
divided the light from the darkness.1:5 And God called the light Day, and the darkness he
called Night and the evening and the morning were the first
day.l :6 And God said. Let there be a firmament in the midst
of the waters, and let it divide
the waters from the waters.1:7 And God made the firmament, and divided
the waters which [were] under the firmament from the waters which [were]
above the firmament and it was so.

1:8 And God called the firmament Heaven. And the evening
and the morning were the second day.1:9 And God said, Let the waters
under the heaven be gathered together unto one place. and let the
dry [land] appear: and it was so.1:10 And God called the dry [land] Earth; and the
gathering together of the waters called the Seas and God saw that
[it was] good.1:11 And God said, Let the earth bring forth grass, the
herb yielding seed, [and] the fruit tree yielding fruit after his kind,
whose seed [is] in itself, upon the earth and it was so1:12 And the earth brought forth grass, [and] herb yielding
seed after his kinds and the tree yielding fruit, whose seed [was] in itself
after his kind: and God saw that [it was] good.1.13 And the evening and the morning were the third
day.... etc.And then were created: sun, moon, evening, day, creatures,
in six days; then God rested.

It is interesting to note that, according to the chapter Genesis, heaven
and earth with the seas were created in one swoop at the very first step,
yet again in the second and third day.

The cooling atmosphereThe knowledge that the earth must once have been red hot, judged by
lava spewing from volcanoes, made people think that the oceans must have
originated from steam in the atmosphere. As the world cooled, it rained
for thousands of years until the oceans were formed, so they thought. But
there's a problem with this idea. The pull of gravity of this planet is
just enough to keep an atmosphere together whose weight is no more than
ten metres of water. So even if the old atmosphere consisted entirely of
steam, the oceans would have filled only to a mere ten metres, rather than
3800. Scientist estimate the amount of water escaping from the early hot
planet at 1.6E9 km3, whereas the volume
of today's oceans is 5.2E9 km3 (W W Ruby,
1951).

Steam from the interiorToday most authors believe that early steam from the hot mantle but
already cool atmosphere, caused the oceans in the very early stages of
the planet. They reason from studies of chondrites (space rocks)
in space that under compression, enough water could be released to form
an ocean. Today one can observe the gases escaping from active volcanoes,
and these too contain water. In this scenario, the oceans would still be
increasing in size, a gradual process that would never really end. But
why did the continental crust expand suddenly about 3 eons ago? (see below)
The amount of water stored in rocks of the primary lithosphere
is estimated at 25E21kg (Hutchinson G E, 1957), whereas the water in all
oceans is 1.35E21kg, so it is quite possible that all this water emerged
slowly after rocks were compressed and heated while the atmosphere had
cooled already.

Debris from spaceFrom the ratios of certain elements, scientists now agree that earth
and the other planets in our solar system were formed by accretion from
interstellar or cosmic debris (dust), left over from the explosion of a
supernova star in our galaxy. Look at the moon and its many craters, each
a remnant of a collision with a large object. The sun gathered most of
it, growing to a size sufficiently large to become a nuclear fusion reactor.
Our planet grew in size and started to heat up due to the falling apart
(radio activity) of heavy elements and due to compression. At one stage
the planet became just liquid enough for lighter materials to migrate to
its surface, and heavy elements to migrate to its core. Continents formed,
and an ocean crust. Knowing that water has been identified on cosmic objects
such as comets, the thought grew that water must have rained down from
space, just like the rest of Earth's mass. Quite a reasonable idea, since
the volume of the earth is 800,000 larger than that of the ocean.

But all the time that the earth formed, it was too hot for an ocean.
In fact, the oldest rocks found are about 3800 million years (My) old (Rocks
from the moon 4550 My), whereas the age of this planet is estimated at
4600 million years. By the time the earth had cooled enough, most of the
cosmic impact that formed it, had also ceased. Why is it that the only
planet with life is also the only planet with an ocean? And why is it that
other planets, that were formed in the same way as ours, do not have oceans?

Co-evolution of climate, oceans and lifeIndependent
scientist James Lovelock, as part of his Gaia hypothesis, made us look
at the world as if it were a single organism. Lovelock observed that life
always changed the conditions of its own existence. Life evolved to make
life better and its environment more suitable for life itself. Organisms
steering the other way would simply not survive. As a result of evolution,
the planet acquired the atmosphere we know now, and an ocean. As part of
its ability to change its environment, life has also acquired primitive
means to regulate the temperature of this planet (green sawtooth line).
Otherwise, due to increasing solar radiation, the temperature would at
least have been 50-60ºC (orange dotted line).

The idea is that a small amount of water formed after cooling (the archean
sea, perhaps 100-300 m deep in small seas here and there, the amount of
water necessary for changing CH4 to C to CO2 and NH4 to N2 and SO4 to S,
and an atmosphere consisting largely of nitrogen, carbon dioxide, methane
and hydrogen - a mixture lethal to organisms alive today. Imagine this
ocean as a shallow, warm, acidic, black cesspool full of stinking bacteria.
The water for this ocean came from the earth's crust, and from sunlight
dissociating CO2 in the upper atmosphere. The formed oxygen could then
bind with the escaping hydrogen to form water. The warm ocean evaporated
much water vapour and it rained torrentially. Because no living organisms
were found on the land, erosion must have been very intense, resulting
in enormous amounts of run-off, containing sediment and nutrients.

For more water to form, the hydrogen, escaping from the planet, had
to be bound with more oxygen. But oxygen was available only as carbon compounds
in carbon dioxide. Only life, in the act of photosynthesis, is able to
split carbon and oxygen from carbon dioxide in large quantities. The carbon
that formed, was buried underground and for each atom of carbon, two molecules
of water were formed. The cyanobacteria capable of doing this, formed about
3600 My ago. By 2500 My ago, the oceans would have formed and the atmosphere
made suitable for higher organisms than bacteria, the prokaryotes. The
light blue band in the diagram is perhaps all it took to create the bulk
of our oceans. By 570 My ago, the Cambrian period started, with plants
and moving animals in recognisable forms. By that time, the atmosphere
had evolved much to what we have today.

As scientists are analysing ratios of elements in ancient rocks, they
are piecing together evidence for the composition of Earth's ancient atmosphere
and its temperature, and with it the answer on how our atmosphere, climate
and oceans formed.

Blue-green algae, also called cyano-bacteria, invented photo-synthesis
some 3600 million years ago. They were also the first living things to
make oxygen, and were the first to learn to breathe it and not get damaged
by oxygen's free radicals. This cyanobacterium is Microcoleus chthonoloplastes,
living on arid shorelines around the world.

Eoastrion (the dawn star), was found in the chert
(silica mineral) of the Gunflint formation. Now extinct, this fossil is
nearly 2000 million years old. It is amazing that such fragile life forms
have been embedded into hard crystals that took a very long time to grow,
and that their forms have been preserved with such detail.

References, available from the Seafriends
Library:Allaby, Michael: Guide to Gaia. 1989.Gribbin, John: Hothouse Earth, the Greenhouse Effect
& Gaia.1990.Joseph, Lawrence E: GAIA, the growth of an idea.1990.Lovelock, James E: GAIA, a new look at life on earth.
1987.Lovelock, James E: The ages of GAIA.1988.Schneider, Stephen H and Randi Londer: The Co-Evolution
of Climate and Life.1984.

Tectonic platesAs
recently as 40 years ago, continental drift became an accepted theory.
Since then, scientists in many disciplines have found overwhelming evidence
of the way continents move.
One of the puzzles was that on the ocean floor, the rocks never seemed
to grow older than about 100 million years, whereas on land they have been
dated back to 3800 million years. Mysterious ridges exist in the centres
of the Atlantic Ocean and the Pacific Ocean. Along 'rims of fire', mountain
ridges and active volcanoes are found. Similar rocks, fossils and organisms
are found on opposite sides of oceans. It wasn't until all these facts
were brought together that scientists realised that the seafloor is gradually
disappearing and that the continents are drifting.

The map shows the tectonic plates relative to the positions of the continents.
The red dotted lines are mountain ranges and rims of active volcanoes.
Here the plates collide, causing mountains to rise and volcanoes to spew
liquid lava. By contrast, along the mid-ocean ridges, the seafloor is spreading,
creating new oceanic crust.

The
diagram shows how oceanic crust is produced in areas where the seafloor
is spreading, while it is pushed underneath continents (subducting) elsewhere.
On a continuous basis new ocean crust is created while thousands of kilometres
further, the crust is subducted and absorbed by the hot mantle.
The Earth's soft mantle consists of hot, semi-liquid stone, which circulates
very slowly by way of convection currents. Where these currents move upward,
undersea volcanic activity occurs and lava pours out, immediately solidifying
in the deep sea. As the crust spreads, it cools further, shrinks somewhat,
breaks and slumps into ridges that run parallel with the spreading zone.
The speed of plate movement is about equal to a growing fingernail
but may differ from place to place (3-15 mm/year). On the ocean floor no
rocks have been found older than 180 million years.

The rocks start to weather and erode, depositing ocean sediment on top
of the ocean crust. Light particles from the land, such as clay are often
blown far out into sea and settle on the sea floor. Sea organisms with
hard skeletons rain down from the plankton-rich layers above. Metals such
as manganese, form nodules on the sea bottom. The ocean sediments are a
mix of many of such ingredients.
In the subduction zone, the ocean crust is pushed underneath the continent,
gradually melting back into the mantle. In the process, sediments from
the land are drawn under too. Due to heat from compression, some of the
material may melt a path through the continental crust, causing volcanic
activity. New continental crust is formed.

The rightmost diagram in above picture shows how both continents and
ocean crust float in balance on the mantle. The continental crust, which
was formed before the oceans, consists of granite, a silica-alumina (SiAl)
rock and is 30-40 km thick. It is lighter (density 2.84) than the mantle
(density 3.27). The weight of the continent equals the displaced weight
of the mantle. The ocean crust, about 11 km thick, is made of silica-magnesia
rock (SiMa) and is heavier (density 3.00) than the continental crust but
lighter than the mantle. It floats also in balance on top of the mantle:
water (density 1.03) + sediment (density 2.30) + crust (density 3.00) =
displaced mantle (density 3.27).

A
new view of Earth's interiorIn order to explain very deep earthquakes 300-600 km
down, scientists have begun to think that the whole mantle is solid. Movements
within it occur due to the rock changing its structure under the influence
of heat and pressure. (The pressure at 100km depth exceeds 30,000 times
atmospheric pressure). The rocky material which comprises the lithosphere
of the earth is special, because the rocks contain water, which is trapped
within the crystal structure of the rocks themselves. These special 'hydrated
minerals' have the ability to slide against each other. On other planets,
where the minerals of the lithosphere do not contain water, the lithosphere
does not flow and hence these planets do not have plates moving on the
surface. As the crust is pushed down into the mantle, the rock
collapses into different crystal structures, producing earthquakes in the
process. Likewise, in convection zones, the rock can expand and 'flow'
in a similar way. The drawing shows the various layers now thought to exist
within our planet.

The heavy iron core is thought to consist of a solid part
in the very centre, surrounded by a molten outer core. The lower mantle,
consisting of oxides and silicates, may also be less liquid than originally
thought. The upper mantle which is more finely layered as shown on right,
consists mainly of magnesium-iron silicates (Mg, Fe)2 SiO4
in various crystal forms.

How is it possible that the inner part of the Earth, thought
to be 4800 degrees C, is solid? At this temperature, iron is well and truly
liquid. The answer must be sought in the two opposing forces that increase
with depth: temperature and pressure: whereas temperature makes stuff more
liquid, pressure does the opposite. The heat in the core is produced by
radioactive heavy elements. For instance, Uranium-238 has a half-life of
4.5 thousand million years (Gy), so about half of it is still left over
since Earth formed. Thorium-232 has a half life of 14 Gy, so the furnace
deep inside the planet will be producing heat for a very long time to come.

Heat radiates out, while cooling in the process. Whereas
heat makes matter liquid, pressure, which increases formidably, tends to
make matter solid. These two opposing forces cause alternating zones of
solids and liquids. A pressure of 3.5 million bar as it exists in the core,
is very hard to imagine. A car tyre contains 2 bar pressure (200 KPa);
a dive cylinder 200 (20MPa); most concrete cracks at 500 bar (50 MPa).

Vertical
movements of continents due to ice agesDuring the ice ages, the ice was in places over one km thick, causing
the continents to sink further into the mantle. After the ice ages, the
continents slowly bobbed up again, a process that takes thousands of years,
and that is still happening today in some places.The map shows clearly
how Canada, Greenland and Scandinavia are still rising from the loss of
the heavy glaciers some 20,000 years ago. In the process of continents
tipping, some places are moving down. For example, Holland is sinking,
which the Dutch are very well aware of.
(Source: W R Peltier & A M Tuschingham, Univ Toronto
in David Schneider: The rising seas, SciAm Mar 1997)

How did scientists puzzle this story together?Nobody has ever drilled a hole into the Earth's mantle (the deepest
drill hole is about 10 km). Much of the structure under ground was gained
from the quest for oil. Geologists detonate explosives to 'ping' the earth,
like a depth sounder would. Arrays of microphones receive the reflected
sound, while computers construct a picture from these signals. Underground
nuclear tests have provided geologists with a wealth of opportunities.
Waves also radiate out from earth quakes and these are very strong at times,
while originating at depths, not reachable by humans. All around the world
a network of seismic stations has been built to record earthquakes and
detonations from nuclear weapons.

It is known that sound travels faster in a denser medium. From measurements
of the speed of sound, the densities of the various layers could be calculated.
Underneath the continental crust, the Mohorovicic Discontinuity or Moho
(Yugoslav geologist Andrija Mohorovicic, 1906) was discovered, marking
the boundary between the solid crust and the upper mantle at about 40km
depth.
In 1926, the German scientist Beno Gutenberg, discovered another discontinuity,
about 150 km deeper down under continents and 70 km under the sea, marking
the boundary with the deeper plastic mantle. It is now accepted that the
tectonic plates extend 70 km deep, the lithosphere (Gk lithos =
stone), which float on a softer mantle, the asthenosphere (Gk asthenes
= weak).

How
did the continents drift?Why the continents suddenly started to drift about 300 million years
ago (The age of Earth is about 4500 million years), is a puzzle (also see
box). Until that time, the continents formed one land mass, named Pangea
(Greek pan, pas = the whole; geo = earth). The first split
between the northern half, now named Laurasia (Europe, North America and
Asia), and the southern half, named Gondwanaland (South America, Africa,
India, Antarctica, Australia and New Zealand), began 300 million years
ago (300Mya). It is possible that the gap between the two continents was
sufficiently large to form the first circumglobal sea, allowing ocean currents
to travel around the world, resulting in a warm equitable climate everywhere.
In any case, the seas that formed between the splitting continents, were
warm and rich in nutrients and marine life, laying down all the mineral
oil we now mine. These changes must have been accompanied with severe climate
changes and changes in vegatation, accompanied by massive erosion. Click
here for a larger version of this map.

On the map one can see how the continents travelled, propelled by ocean
plates that have changed shape enormously. The red curves are areas of
collision, whereas the blue lines are those where the seafloor was spreading.
Where the continents collided, high mountain ridges were formed, such as
in Europe (colliding with Africa): The Pyrenees, the Alps and more. An
interesting case is India which travelled all the way north to collide
with Asia, forming the Himalayan mountains. Australia broke away from Antarctica
at a later stage, pulling New Zealand with it. But how did scientists puzzle
this story together?

The secret lies in a weak property of rock: its magnetic field. Inside
many rocks is found the element iron, a very common element on this planet.
Iron and some of its oxides can be magnetised by the magnetic field of
Earth. This won't happen as long as the rock is liquid, but by the time
it cools sufficiently to become rock, the Earth's magnetic field is 'frozen'
in place. Geologists drill deep sampling cores and analyse the magnetic
field orientation. To make matters worse and easier at the same time, the
magnetic poles have reversed several times and they have been wandering
around somewhat. So the data needs to be corrected by what is known about
the oscillations in the Earth's magnetic field. But at the same time, the
field reversals are also convenient time markers to age the layers in the
core samples.

The
continents may have been drifting for much longerNew evidence with respect to tectonic plate movements,
suggests that the continents may always have been drifting apart, then
together again. When continents collide, they form folded mountain ranges.
A number of such ranges could be explained only by assuming that the continents
have collided once or twice before. A difficulty in studying old mountain
ranges is that they have all but completely eroded away. But as scientists
are drilling more holes, their knowledge pieces together the continental
crust movements dating back to 500 million years.In the drawing of the world map with all continents joined
together, the red mountain ranges could be explained by subduction of ocean
plates, uplifting the continents at their margins. Yet some of these ranges
are folded extensively. The purple ranges are all folded and are thought
to have occured by the continents colliding towards the Pangea configuration.
On left the mountain range from Ouachita belt, through Appalachian belt
and the Caledonian belt over Iceland, Scandinavia and Greenland. The Hercynian
belt runs through the Pyrenees and the Alps. The Uralian belts criss-cross
through Asia and Siberia.

This
map shows continental movement back to 500 Mya, where the history of Pangea
began, when continents were dispersed around the Iapetus Ocean. About 400
Mya, Laurentia (America) collided with Baltica (Europe) to form the super
continent Laurasia. In the process the Apalachians, Iceland, Scandinavia
and Greenland mountain belts were formed. The Iapetus Ocean vanished when
Laurasia fused with Gondwana (all the other continents), creating Pangea
(300 Mya). The mountain belts of the Atlas were formed and the Urals between
Siberia and Europe and the many mountain belts in Asia. The plate motions
changed as Pangea dispersed. North America moved north, then west away
from Eurasia (180 Mya). In a cycle of 500-600 My, the continents may have
been dancing to and fro for over 1500 million years. As scientists gather
more data, this puzzle may be pieced together more accurately. (See also
the
Geologic Time Table) (Source: J Brendan Murphy and R Damian Nance: Mountain
belts and the supercontinent cycle, Sci Am Apr 1992 p34-41

A
possible mechanismBut what could the mechanism for such movement be? A
simple explanation is shown in the four drawings on right. It is generally
accepted that the crustal movement is caused by convection currents, originating
from differences in temperature between spots in the liquid mantle. The
Earth cools its interior by leaking heat to the outside. Where the crust
is thin, such as underneath oceans, heat is lost more easily, resulting
in a cooler area. But since continents are four times thicker, they also
insulate better. A hotspot could appear underneath a large continental
slab. As the solid mantle heats up, the coninent is bulged up and cracks.
It allows a convection zone to form, pushing the broken halves apart and
ending up as a spreading mid-ocean ridge. At some other place on the globe,
continents ae pushed together again, and a new hot spot is formed, while
the old hot spot shrinks. Continents break up again and reverse their travel.

Growth
and formation of continental crustOne of the remaining problems is that land erosion happens
so fast, that all the continents should have disappeared below sea level,
a very long time ago. Particularly in the azoic (no-life) era (till 2.5
eons ago) without any cover on the land, erosion would have been very high.
By measuring the very low concentrations of the rare-earth elements (EER
or Lantanides, see the Periodic Table of
Elements), geologists discovered that the sedimentary rocks, originating
from mud washing into the oceans, represent a very good average of the
composition of the continental crust. The oldest mineral (zircon) is found
in sedimentary rock in Australia and is 4.2 eons old. In north-west Canada,
the Acnasta Gneiss formation yielded the oldest rock (granite) that was
not formed from sediment.

Now that a very large number of measurements have been
made on sedimentary rocks of various ages, the actual volume of the continental
crust can be plotted. In the figure on right, the bottom graph shows how
the volume of crust has been growing, sometimes slowly, sometimes more
rapidly. The time scale is in eons (thousand million years). Almost immediately
after Earth accreted from particles and meteors, a small amount of crust
was formed and during the first eon, this amount did not increase very
much (erosion was very high). Only when the ocean plates started moving,
was continental crust formed, first fast because the interior of Earth
was hotter, then more slowly. The crust that formed in the first episode
of fast growth (granite?), is different from later rocks, perhaps due to
the possibility that oceanic crust was recycled much faster than today.
(due to the presence of a deep ocean??).

The top diagram shows a slab of oceanic crust subducting
under a continent. By pushing sediments up, the continent grows from accretion.
It also acquires more volume from volcanoes. It is now thought that sedimentary
rock melts, not only from friction but mainly from the amount of water
it contains, which acts as a flux additive in a foundry, inducing first
the sedimentary rock to melt and then the continental rock as it moves
upward to the surface. Magma chambers do not always reach the surface but
can convert to granite, which is included as part of the continental crust.

Current oceanic crust forms mainly by the eruption of
basaltic lava along a globe-encircling network of mid-ocean ridges. More
than 18 million cubic km of rock are produced this way, each year. The
simple concept of continents made from granite, formed when the mantle
was liquid, while staying the same size, needs to be changed in the light
of these new findings.

Note that the first rapid growth of continental crust
suggests that oceans were formed just before 3 eons ago.

Naming
the features of the ocean basinsIn this drawing of a typical ocean profile, the vertical scale has
been exaggerated, to better illustrate the ocean's shape.Where the land
meets the water, continents start with a gradual slope down to about 200m
depth. It is called the continental shelf. For various reasons, this is
the most productive part of the ocean. Beyond the continental shelf, the
seafloor dips down more steeply (the continental slope) until it becomes
more gradual again (the continental rise). Some continents are flanked
by deep trenches. Where the continental rise ends (a vague boudary), the
abyssal plane starts (Gk
a = no; bussos = depth; bottomless).
Where the ocean's floor is spreading, an ocean ridge is found, flanked
by large fracture zones.

A three-dimensional map of the oceansEver since people were sailing the oceans, attempts have been made
to map them, but the deep oceans, often exceeding 5000m depth, were impossible
to plumb until electronic depth sounders were invented. Two scientists
in particular, gathered together the sounding data and drew up new and
historic maps of the sea floor (see below). In the International Geophysical
Year, scientists from all nations made a concerted effort to co-operate
in their study of the planet. Since then, the study of our oceans has been
pursued with rigour.

The ocean's floor, painstakingly mapped by Bruce Heezen
and Marie Tharp, just after World War II.The spreading zones can clearly be distinguished from
the subduction zones with their trenches.

Ocean floor mapping from space?In 1978, an experimental satellite was launched to study the oceans.
One of Seasat's instruments was a radar altimeter, able to measure the
ocean's surface from an altitude about 500 miles up, with a precision of
5-10 cm. Surprisingly, the surface of the ocean proved to be curving and
dipping, often by as much as 10m up and down (for 5000m bottom relief).
When mapped, the dips corresponded to dips in the sea floor and the bumps
to sea mounts and ridges. The precision of the measurements allowed computers
to make precise and detailed maps of the entire world's ocean floors.

The principle that causes the unevenness of the ocean's surface, is
gravity. A water particle is pulled towards the centre of the Earth by
its gravity but when it is close to a sea mount, it is also pulled towards
the sea mount's mass. As a result, water is piled up above sea mounts and
ridges and dipped down above trenches. Seafarers have never been able to
notice this because it does not affect the ship's course, nor can it be
seen or felt.

Although the Seasat satellite was launched in 1978, to
study the sea, it wasn't until 1982 that its datawas used to map the height of the ocean's surface. It
revealed in astonishing detail the contoursof the ocean's bottom, with sea mounts and ridges that
had not been discovered before.